1. Field of the Invention
The invention relates to an optical information storage medium and a method for fabricating the same in which information can be recorded and reproduced rapidly and at a high density by using optical means such as a laser beam. In particular, the invention relates to a phase change type optical disk and a method for fabrication the same.
2. Description of the Related Art
The technique of recording and reproducing information at a high density by using a laser beam is known in the art, and is put to practical use mainly in the form of optical disks.
Optical disks can be roughly classified into a read only type, a write-once type, and rewritable type. The read only type optical disk is practically used as a compact disk or a laser disk. Optical disks of the write-once type and of the rewritable type are practically used as document files, data files or the like.
The rewritable type optical disk can be classified into a magneto-optical type and a phase change type.
The phase change type optical disk utilizes a phenomenon in which a state of a recording layer reversibly changes between an amorphous state and a crystal state (or between two crystal states having different structures from each other) by irradiation of a laser beam. When a laser beam irradiates, at least one of the refractive index and the extinction coefficient of the recording layer changes, whereby signals are recorded. In the portion where the state of the recording layer changes, the amplitude of light transmitted therethrough or reflected therefrom changes and as a result, the quantity of transmitted or reflected light which can reach a detection system changes. This change in quantity of the light is detected so as to reproduce the signals. Materials which show the reversible change of the state between the amorphous state and the crystal state are alloys of Te, Se, In, Sb or the like.
The phase change type optical disk is fabricated by laminating a dielectric layer, a recording layer, a reflecting layer and so on on a substrate by a vapor deposition method or a sputtering method. Compared to the vapor deposition method, the sputtering method provides better adhesion properties between the substrate and the deposited layers and higher reproducibility of thickness and composition of the deposited layers. In addition, larger and more uniform deposited layers can be obtained by sputtering. For these reasons, the phase change type optical disk is mainly fabricated by the sputtering method. Moreover, in the conventional sputtering method, cheap Ar gas is usually used as a discharge gas.
However, the phase change type optical disk fabricated by the above conventional sputtering method using Ar gas has a problem in that the number of recording/erasing cycles is limited because of deterioration phenomena caused by repeated overwrite operations. One of such deterioration phenomena is aggregation and growth of voids in the recording layer caused by repeated overwrite operations, with the result that there arise problems such as decrease of signal amplitude and increase of noise level.
It is suggested that there are the following two causes of void growth. First, Ar gas introduced into the recording layer during a forming step thereof by sputtering gradually separates and aggregates so that the Ar voids generate and grow (cf. Horie et al., Proceedings of the 3rd Symposium on Phase Transition '91 autumn, P.7). Secondly, Ar gas introduced into the dielectric layer during a forming step thereof by sputtering gradually separates from the dielectric layer by repeated overwrite operations and are introduced into recording layer so that the Ar voids generate and grow in the recording layer.
The second cause will be described in more detail as follows. During the recording/erasing processing of the signals, the temperature of the recording layer of the phase change type optical disk rises up to about 600.degree. C. which almost is equal to the melting point of the material of the recording layer. As a result, the dielectric layer which is adjacent to the recording layer is also heated to a high temperature. This heating causes Ar gas to separate from the dielectric layer and to enter the recording layer. The Ar gas once introduced into the recording layer aggregates to generate the Ar voids in the recording layer.
The above-mentioned void growth which is caused by Ar gas being used as a discharge gas in the forming steps by sputtering and the related deterioration of performance properties of the phase change type optical disk are well-known phenomena. However, no substantial solutions for the problem are disclosed so far. Changing sputtering conditions, such as gas pressure, does not significantly improve the performance properties of the phase change type optical disk.